1. Field of the Invention
The invention relates to a powder for molding that affords enhanced moldability (in particular, reduction in ejection force) while requiring less internal lubricant, to a lubricant-concentrated powder that is used to prepare a powder for molding and that is made up of metal base particles on the surface of which an internal lubricant is adhered at a high concentration, and to a method for producing a metal member, which is molded compact or sintered compact thereof, and obtained using the powder for molding.
2. Description of Related Art
Metal members of complex shape are produced by way of molded compacts resulting from pressure-molding a starting powder (powder for Molding) that fills the cavity of a die, and by way of sintered compacts resulting from heating the molded compacts. Such a production method allows significantly reducing production costs of the metal member, by, for instance, reducing cutting processes.
In order to produce stably high-quality metal members in accordance with such a method, it is important that the molded compact be removed smoothly, with a low ejection force, without occurrence of galling, seizure or the like between the starting powder or the molded compact and the inner wall surface of the cavity of the die, during pressure-molding of the starting powder and during ejection of the molded compact. Such being the case, internal lubricants have come to be added to and mixed with starting powders. The greater the addition amount of the internal lubricant, the more internal lubricant can be supplied at the boundary between the starting powder or molded compact and the inner wall surface of the die; accordingly, it is deemed that this may allow suppressing the occurrence of galling and the like during pressure molding and during ejection.
Internal lubricants, however, are fundamentally added merely with a view to enhancing moldability, and do not contribute to enhancing the characteristics of the metal member, but rather give rise to decreased density of the molded compact, and to increased porosity (lower pore-free density (PFD)). A greater amount of internal lubricant translates into a longer removal process (dewaxing process) of the internal lubricant, as required during sintering of the molded compact. Therefore, essentially the addition of the internal lubricant is implemented at an amount as small as possible.
Various approaches have been proposed, in the light of the above considerations, for reducing the amount of internal lubricant while suppressing the occurrence of galling or the like during molding. In this regard, the related art literature discloses for instance the features below.
Japanese Patent Application Publication No. 1-219101 (JP 1-219101 A) discloses the feature of performing warm molding at the temperature at which an internal lubricant melts, to cover thereby completely the surface of iron powder (starting powder) with comparatively little internal lubricant. The addition amount (total amount) of the internal lubricant as disclosed in the examples of JP 1-219101 A, however, is 1 mass %, which does not constitute a sufficient reduction in the amount of internal lubricant.
Published Japanese Translation of PCT application No. 2001-524605 (JP-A-2001-524605) discloses examples where the addition amount of internal lubricant is 0.6 mass %, which still does not constitute a sufficient reduction, as in the case of JP 1-219101 A.
Japanese Patent Application Publication No. 2009-523907 (JP 2009-523907 A) discloses an example where the addition amount of internal lubricant is reduced down to 0.4 mass %. In JP 2009-523907 A, however, a special metal powder is used wherein particles are covered beforehand with a metal salt of phosphoric acid, in order to reduce the addition amount of internal lubricant. Even using that special metal powder, the addition amount of internal lubricant can only be reduced at most down to 0.4 mass. Both JP-A-2001-524605 and JP 2009-523907 A use a starting powder that results from uniformly mixing a particulate internal lubricant into a metal powder.